From a range of modern display devices, like liquid crystal display devices (LCD devices), crucial data is needed to be retrieved by the human eye. These modern display devices are more important than ever before for a large number of applications. For instance these can be medical imaging devices, displays in control rooms, professional broadcast monitors, defense and avionics displays etc. As a result high-quality imaging using display devices like liquid crystal display devices (LCD devices) is essential.
In the case of medical imaging, but not limited to, suitable display devices are typically provided with a sensor and a controller device coupled thereto. One type of sensor is coupled to a backlight device, for instance comprising light emitting diodes (LEDs), of the LCD device which has the objective of stabilizing the output of the backlight device, which inherently varies as a consequence of the use of LEDs therein. Another problem to overcome when diagnosing using a display device is that these medical displays can be used in environments, which have a relatively high ambient light level, depending on the exact application. This ambient light level directly impacts the perceived contrast of the display (because typically the display partially reflects impinging light), which again impacts the diagnostic performance. EP1274066B1 discloses a display device wherein the sensing is applied in front of the display, comprising a light guide, for instance a waveguide or fiber, to guide a portion of the light output to a sensor outside of the viewing angle of the display. Light originating from a display area comprising a plurality of pixels is introduced into the light guide, for instance into a continuous waveguide or at one end of the fiber. In addition, the portion of the display that is blocked for light transmission is limited. Particularly light rays traveling under a large angle to the axis of the light guide can be forced to exit the structure, while ambient light is obstructed from entering the light guide. By using this small acceptance angle, entering of that ambient in the photodiode sensor without a need for shielding is realized. However, it is desired to further improve such a sensor system, i.e. sensor and light guide. For instance by using one implementation shown in EP1274066, wherein an end of a fiber is parallel to the output surface of the display and the fiber is bent. This is however not a most practical implementation. WO2010/081814 discloses a display device comprising a sensor system with at least one sensor for detecting the intensity or color of light emitted from at least one display area of the display device into the viewing angle of the display device. The sensor system further comprises an at least partially transparent sensor and in another embodiment at least one optical coupling device including a light guide member and a incoupling member for guiding or directing, e.g. deflecting, reflecting, bending, scattering, diffracting, at least one part of the light emitted from the display area to the corresponding sensor, wherein the latter sensor is located outside or at least partially outside the viewing angle of the display device.
However in an environment with a certain level of ambient light (i.e. light not originating from the backlight of the display), the measured light emitted from the display area to the corresponding sensor will be a combination of the light emitted by the display and the ambient light falling on the sensor from the environment. This may dynamically change for instance due to shadows being cast on the display screen. It would thus be beneficial if measurements are performed using the described display and in addition to adapt the backlight of this display to the amount of measured ambient light in order to maintain perceived contrast within tolerable limits.